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This invention relates to quadrature amplitude modulation systems in general and particularly to a system in which signal components are modulated unto a subcarrier to allow for inclusion of carrier and bit sync information at the center of the transmitted frequency spectrum. In a quadrature modulation system, designed for radio transmission, it is desirable to transmit a pilot carrier in order to facilitate reception and decoding of the transmitted information. While it is known to transmit a pilot carrier, such carriers have been added as a side frequency to the quadrature amplitude modulated signal. There are, however, limitations and disadvantages to this approach. Due to selectively fading, such as can occur on a radio path, problems can result where the pilot carrier is spaced from the signal information. Additionally, problems can occur due to limitations in the passband of receivers i that the pilot carrier signal may fall too close to or outside of receiver's passband filter edge. Consequently, it is desirable that such a pilot carrier be located more centrally to the transmitted frequency spectrum.
Where digital information is to be transmitted in a synchronous manner, channel fading and noise can prevent the maintenance of bit sync. In such a system it is therefore desirable to transmit a bit sync signal with the synchronous digital information. Like the pilot carrier, the bit sync signal should be provided as close to the center of the transmitted frequency spectrum as possible.